Historically, an enterprise (such as a business, government, or non-profit entity) used a private branch exchange (PBX) to provide conventional land-line telephone service within locations that are controlled by the enterprise. If the enterprise needed to provide mobile telephone service to its employees, the enterprise would use a public cellular telephony service to provide cellular telephone service to its employees using mobile handsets. Wherever the employee used the mobile cellular handset (including within the enterprise), the associated service provider's home public land mobile network (HPLMN) would be used to provide cellular service to the cellular handset (with the associated costs).
Fixed-mobile convergence (FMC) solutions attempt to leverage an enterprise's wireless data network to provide mobile telephony service in locations controlled by the enterprise. FMC solutions typically employ a dual-mode mobile handset. Such a dual-mode handset operates in a cellular mode in which the handset is used to provide cellular service using a corresponding HPLMN and operates in a VOIP mode in which the handset is used to provide voice over Internet Protocol (VOIP) service using a wireless local area network. However, such dual-mode mobile handsets are typically special devices (that is, are not regular cellular mobile devices) or implemented using special client software that must be installed on a regular cellular mobile device. Also, the corresponding HPLMN typically must be modified to support such a dual-mode handset (in particular, to support handovers between the cellular network and a wireless local area network).
Moreover, typically when such a dual-mode handset is operating in VOIP mode (that is, when the dual-mode handset is registered with the wireless local area network and is ready to receive or make VOIP calls), the dual-mode handset appears to the corresponding HPLMN to be offline even though the handset is on and operating in VOIP mode. Likewise, when such a dual-mode handset is operating in cellular mode, the dual-mode handset appears to the corresponding VOIP system to be offline even though the handset is on and operating in cellular mode.
Another communication technology that is increasingly being deployed in enterprises is unified communications (UC) technology. Typically, unified communications technology is deployed in order to integrate various different enterprise communication technologies such as VOIP and legacy telephone service, email, audio and video conferencing, voice mail, presence and contact information, faxes, instant messaging (IM), and calendaring. One way in which UC is implemented employs one or more UC servers (such as Microsoft Exchange Server 2007 and Microsoft Office Communications Server 2007, both of which are commercially available from Microsoft Corporation). Efforts to integrate cellular mobile devices into such UC solutions, however, have typically involved providing remote access to communication resources (such as email, calendaring, and voice mail) via a smart phone type of mobile device using a public cellular data network. Such efforts have not, however, integrated traditional cellular voice service into the overall UC solution, nor have such solutions integrated presence information related to such cellular mobile devices into the UC solution.